1. Field of Invention
This invention relates to extruders and injection machines of the type in which a screw rotatable within a barrel is employed to extrude or inject molten resinous material to the outlet port of a plasticity barrel. More particularly, this invention is concerned with thorough melting and mixing resinous material at the end of the transition and into the metering section of the plasticity screw.
2. Background Description of Prior Art
Plasticity equipment commonly used today are of the type which receive polymer pellets or powder, heat and work the polymer to convert it into a melted or molten state before delivering the molten polymer under pressure through a restricted outlet or discharge port. Although there are several different types of plastic polymers each having different physical properties, it is desirable that the extrudate leaving the typical plasticity equipment be fully melted, homogeneously mixed and uniform in temperature, viscosity, color and composition.
The plasticity apparatus induces an elongated cylindrical barrel, which may be heated at various locations along its axial length, and the screw which extends longitudinally through the barrel. The screw has a core with a helical flight thereon and the flight cooperates with the cylindrical inner surface of the barrel to define a helical valley or channel for passage of the resin to the plasticity apparatus outlet port. Although the pitch of the flight can vary it is common to utilize screws of constant pitch. The pitch is the forward distance traversed in one full revolution of the flight. It is also common that screws have a pitch distance that is equal to the outside diameter of said screw. Although there are different screw configurations for different polymer compositions, the typical plasticity screw ordinarily has a plurality of sections along its extended axis with each section being designed for a particular function. Ordinarily, there is a feed section, a transition section and a metering section in series. The plasticity screw feed section extends beneath and forward from a feed opening where a polymer in pellet, powder or regrind form is introduced into the plasticity apparatus to be carried forward along the inside of the barrel. While being carried along said screw axis, the polymer is absorbing heat from said heated cylinder. The depth of said helical flight of the screw in the feed section is usually large enough to overfeed the solid polymer. The overfeeding action serves to compact and pressurize the polymer particles and form a solid bed of advancing material in the plasticity apparatus.
The material is then worked and heated in the transition section so that melting of the polymer occurs as the material is moved forward along said screw axis toward the outlet port. The polymer is passed through the transition section to reduce the root depth of the helical passageway to reflect the volume reduction due to the melting of the feed. The reduction of depth in the transition section also compresses the solid bed of pellets or powder. The transition section leads to a metering section, which has a shallow root depth helical passageway. The preferred geometry moving from the deep feed section to the shallow metering section takes the form of an involute taper geometry. The metering section has as its function the exertion of a constant flow rate pumping action on the molten polymer. In addition, any unmelted solids should be melted in the metering section as well as to mix the melted polymer homogeneously. It is understood that a polymer cannot be mixed properly until it is first melted. Generally, when the metering section begins, if the change in flight depth from feed to metering is sufficient and the length of transition sufficient, the resin is at least 90 percent melted. As previously stated and as described in U.S. Pat. No. 4,752,136, the root depth of the metering section is generally shallow. This shallow depth increases the shear and friction in the polymer, which has a tendency to raise the temperature of the polymer urging the remaining solids to melt. An increase in shear rate and temperature usually has a substantial effect on the viscosity of the polymer. A change in viscosity of the material being plasticated in turn affects the flow rate of the material through the restricted outlet port. As a result, without the optimum screw configuration, there may be a failure to achieve the desired uniformity and output rate of molten polymer, which is a significant problem for the plasticity operation. What makes this task even more difficult is that the current state of the art challenges us with length to pitch and diameter ratios of 12:1 to 27:1. When there is a demand for a high output rate there is frequently polymeric material without the addition of an expensive, complicated mixing section that is typical of advanced screw designs.
To my knowledge there are no designs that make an attempt to increase the flight pitch to achieve a shallower flight depth while maintaining a similar channel volume, in addition there is no relationship between the flight and the root like the present invention employs.
The present invention is directed to a screw configuration, plasticity apparatus and method for improving melting and mixing of resinous material in the metering section.
In accordance with this invention a plasticity screw having a feed section, transition section and a metering section in series, the flight pitch normally being constant in the feed and transition sections, each section having a flight channel forming a specific channel volume when compared with each other forms a compression ratio with the volume of the feed section being greater than that of the metering section. Said screw having at least one but preferably two or more changes in the flight pitch and root diameter at the end of the transition section and through the metering section with the root stepped in cooperation with an increase in flight pitch. Said changes that are dependent upon the flighted length to screw diameter ratio, screw diameter and resin composite. not enough axial length to accomplish all that is desired and a compromise in melt quality is experienced.
It is desirous to have a metering section with a shallow flight depth so as to assure that there is a substantial shear rate and good conductive heat transfer from the heated cylinder to the polymer. The most effective melting mechanism takes place between the heated barrel and the polymer. When using the state of the art constant pitch compression screw design that is the most prominent screw in use today, a shallow meter depth can only be accomplished with a high compression ratio between said feed helical channels and said metering helical channel. The practice of increasing the taper to achieve a shallower flight depth in the metering section is proven to be counter-productive because as the taper increases the effective melting length of the screw decreases. In addition, a high compression ratio cannot be used with many polymer types because of excessive shear rates therefore this practice is limited in its scope.
Our invention extends the melting length by utilizing a low volumetric compression ratio. In addition, by increasing the flight pitch of the screw at the end of the transition section as seen in FIGS. 2 through 3, the polymer is exposed to more barrel wall surface area and excellent heat transfer. The increase in the flight pitch also increases the velocity between the barrel and the polymer adding a melting and mixing effect. And lastly, by stepping the root in cooperation with the flight pitch change, an added degree of mixing is achieved. So, one skilled in the art would surmise that our invention has created a design that generates a more thoroughly melted and mixed A description of the preferred geometry of the invention follows.
1). The flight depth and pitch of the screw are used to calculate a channel volume. The screw is designed to achieve a given ratio between the channel volumes in the feed section vs. The metering section. At the end of the transition section a tangent point or tapered terminus is reached along the axis of the root that represents a flight depth. The flight depth at that tangent point is or tapered terminus used to calculate a channel volume. A substantial increase of the flight pitch takes place, the purpose being to achieve a similar channel volume with a shallower flight depth. Said change in the flight pitch should be at least 1.25xc3x97(times) the pitch used through the feed and transition sections and preferably be about between 1.25 and 1.50. An increase in the pitch or helix angle of the flight has the effect to expose a greater amount of polymeric material to the barrel wall as well as to increase the relative velocity between the barrel and the resin. It is well known to those skilled in the art that the most effective melting occurs between the hot barrel and the polymeric material. The resultant shallower than normal flight channel depth has a greater ability to melt the resin completely because of a higher shear rate and more efficient conductive heat transfer even though the volumetric compression ratio remains low which assures a longer axial length of melting ability. The change in velocity has the effect to add a degree of melting and homogenous mixing to the hot resinous material.
2). In the invention, the increase in pitch occurs in conjunction with a change in the root diameter, so as the flight pitch increases, the root of the screw transitions from a deep flight depth to a shallow flight depth. The stepped change in the root that starts at the same tangent point or tapered terminus as the increase in pitch ends preferably about between 0.7 to 0.9 or 1.1 to 1.3 times the length of the increased flight pitch, namely upstream or downstream of the end of one complete revolution of the flight.
3). An option that executes two flight pitch increases in succession that are separated by a constant depth metering section, said first flight pitch is about between 1.20 to 1.30 times the original flight pitch, said second flight pitch is about between 1.35 to 1.50 times the original flight pitch. The first change in the root that starts at the same tangent point or tapered terminus as the increase in the first pitch ends preferably about between 0.7 to 0.9 or 1.1 to 1.3 times the length of the increased flight pitch, namely upstream or downstream of one complete revolution of the flight. The second change in the root ends preferably about between 0.7 to 0.9 or 1.1 to 1.3 times the length of the increased flight pitch, namely upstream or downstream of one complete revolution of the flight. It is understood that multiple changes in the flight and root profile while subjecting the resinous material to our substantially shallower metering flight depths are good for melting and mixing.